Collagen is the most abundant protein in mammals, constituting a quarter of their total weight. Collagen provides the tensile strength in the connective tissues of all animals and is the major fibrous element of skin, bone, tendon, cartilage, blood vessels, and teeth.
Collagens are classified into several types based on sequence identity and function. Types I, II, & III collagen molecules make up the main fibers of most animal extracellular structures. Type I forms about 90% of the body's collagen and is the primary component of bone, skin and tendons. Type II makes up the major fibers of cartilage. Collagen fibers are arranged in rigid plates in bones, in parallel bundles in tendons, and in a dense meshwork in cartilage. Type I and lesser amounts of type III make up tendons and skin. Type IV collagen molecules make up very fine, unstriated fibers present in basal laminae. Over a dozen other collagen types are known but are less well characterized.
Collagen polypeptide chains are characterized by a core helical domain made up of repeating glycine-X-Y triplets and globular N-terminal and C-terminal domains. Three such chains are wound around one another in a superhelix to generate an individual ropelike collagen molecule.